1. Field of the Invention
This invention relates to an objective lens for use in an optical memory device for writing and reading by a laser and, more particularly, to an objective lens for an optical memory device having preferable chromatic aberration and a plenty of utility.
2. Description of the Prior Art
An objective lens having an aspherical surface on one side and a plane surface on the other side for use in an optical disk device is disclosed, for example, in Japanese Patent Application Laid-open No. 88213/1986. If a refractive index is determined in the lens having an aspherical surface on one side and a plane surface on the other side of this type, its central thickness and a variation in its aspherical surface are automatically determined when its sine conditions and spherical aberration are corrected.
In the objective lens disclosed in said Japanese Patent Application Laid-open No. 88213/1986, its refraction index n has a condition of n&gt;1.68. However, assume, for example, that the lens has a condition of n&gt;1.68, the thickness of the periphery of the lens becomes excessively thin, and, when it is adhered with an adhesive, there is a possibility that a distortion is affected in an effective diameter from the periphery of the lens.
Further, in the conventional objective lenses including the lens disclosed in the above-described laid-open application, a countermeasure for its chromatic aberration is not considered in case of manufacturing the lens.
In an optical memory device, an emitting light power in case of writing information on a disk and an emitting light power in case of reading information from the disk are switched. Normally, the emitting light power in case of writing is approx. 30 mW, and the emitting light power in case of reading is approx. 3 mW. However, in a semiconductor laser realized at present, a variation in the wavelength of approx. .+-.5 nm occurs upon switching of the emitting light power. When the wavelength is varied in this manner, a variation in its focal distance occurs due to the difference of the wavelengths in case of switching from the reading to the writing or vice versa, and its focus servo is offset so that there arises a problem that the servo becomes impossible in operation.